The overall goal of this application is to study cerebral microvascular vasodilator function in young and old humans and test hypotheses broadly related to microvascular function and cognition. This project will serve as a vehicle to build upon the applicant's training in cardiovascular aging and integrate with the cerebrovascular changes associated with Mild Cognitive Impairment (MCI) and Alzheimer's disease. Recent evidence suggests that microvascular dysfunction is a major contributing factor to MCI, the transitional stage between normal aging and dementia or Alzheimer's disease. Because Alzheimer's patients demonstrate systemic endothelial dysfunction and reduced endothelial progenitor cells (EPCs), it is likely that MCI patients have a similar phenotype. To address our overall goal we will systematically evaluate microvascular vasodilator function in humans with and without MCI. In Aim 1 we will determine if cerebral and peripheral vasodilator responses are blunted in MCI patients by comparing CO2-mediated vasodilation in the middle cerebral artery (MCA) and the forearm blood flow response to reactive hyperemia in 3 groups: 1) young healthy adults; 2) cognitively intact older adults; and 3) older adults with MCI. In Aim 2 we will determine if a loss of vasodilating prostaglandins explains the reduction in MCA vasodilator responses to CO2 by comparing the effects of indomethacin on CO2-mediated vasodilation in the MCA in the same 3 groups. We expect a loss of prostaglandin-mediated vasodilator mechanisms in both the intact older subjects and MCI patients. In Aim 3 we will determine if blunted EPC regenerative capacity accompanies the reduction in cerebral vasodilator responses to CO2. We will determine the regenerative capacity by assessing reendothelialization using a carotid artery injury mouse model after transplantation of human EPCs from 3 groups: 1) young healthy adults; 2) older cognitively intact adults; 3) older adults with MCI. We expect that EPC regenerative capacity is greatest in young subjects, intermediate in older adults and reduced in MCI patients and correlated with CO2-mediated cerebral vasodilation. In summary, we have a comprehensive plan to investigate cerebral vasodilator function in humans and how it is altered by age or cognitive status. The use of a stepped hypercapnic ventilatory protocol will permit us to assess graded vasodilator responses in the MCA in a way that is non-invasive yet conceptually similar to the tests of vascular function in the forearm and heart that have provided great mechanistic understanding of microvascular pathology in cardiovascular disease. The studies with indomethacin will permit us to test the role of vasodilating prostaglandins in the loss of vasodilator function with aging and MCI. The examination of EPC regenerative capacity further explores the notion that microvascular dysfunction is an underlying factor in MCI. Together this approach will provide significant insight into the role of microvascular dysfunction in cognitive impairment and permit the candidate to develop translational expertise in the mechanisms of cerebral blood flow regulation.